1. Field of the Invention
The present invention relates to a unidirectional mode converter and an optical isolator that uses the unidirectional mode converter and which forms an essential element in optical communication systems and other such optical applications systems.
2. Prior Art Statement
Conventional optical isolators have been formed of glass for visible region types and yttrium-iron-9arnet single crystal for near-infrared types. For conventional communication systems, waveguide type optical isolators are considered to be most advantageous in terms of compactness and low cost.
The conventional basic arrangement and working principle of a waveguide isolator will now described with reference to FIG. 3. Essentially, this type of optical isolator is a unidirectional optical path along which the light is allowed to propagate within a waveguide path 2 formed on a substrate 1 in a prescribed direction, for example from the input plane 3 on the left towards the output plane 4 on the right, but cannot travel in the reverse direction.
To achieve this in the conventional arrangement, as in the optical isolator shown in FIG. 3, the waveguide path 2 is an optically permeable magneto-optic thin film in which first and second regions a.sub.1 and a.sub.2 are formed in tandem along the direction of propagation of forward input light Ii entering from an input plane 3. In the first region a.sub.1 the Faraday effect observed when the direction of magnetization f.sub.1 is parallel to the light propagation direction causes the input light Ii to undergo non-reciprocal mode conversion, and in the second region a.sub.2 the Cotton-Mouton effect observed when the direction of magnetization f.sub.2 is perpendicular to the light propagation direction causes the input light Ii to undergo reciprocal mode conversion. Also, formed on the waveguide path 2 in the vicinity of the input and output planes 3 and 4 are transmission mode polarizers or mode selectors 5 and 6 comprised of metal cladding layers.
The basic operation of this optical isolator is as follows. The forward input light Ii enters the waveguide path 2 via the input plane 3 and via the mode selector 5, which transmits only the TE-mode component, enters the non-reciprocal mode conversion first region a.sub.1 where the polarization plane is rotated 45 degrees. However, as this rotation is canceled by the following reciprocal mode conversion second region a.sub.2, the input light Ii reverts to the TE mode and can therefore be output from the output plane 4 as forward output light Io.
The mode selector 6 located by the output plane 4 causes undesirable optical noise Ii', such as reflected output light Io, for example, to be transmitted to the waveguide path 2 as TE-mode light, which undergoes TM-mode conversion from its passage through the reciprocal mode conversion second region a.sub.2 and the non-reciprocal mode conversion first region a.sub.1, thereby preventing it from passing through the mode selector 5 or, if the mode selector 5 is metal-clad, is absorbed, but whichever the case the result is that output of this optical noise from the input plane 3 is suppressed.
From the above explanation, it can be seen that without the mode selectors 5 and 6 the arrangement would function as a unidirectional mode converter.
Thus, in the above conventional optical isolator, the isolation of the output light Io from the noise component Ii' can be achieved satisfactorily provided the selectively non-reciprocal and reciprocal mode conversion regions a.sub.1 and a.sub.2 can be readily formed in the magneto-optic thin film which constitutes the waveguide path 2. In practice, however, such tandem formation of these regions a.sub.1 and a.sub.2, which have different magnetization directions, is extremely difficult, requiring the use of large, complex, costly magnetizers, and is a particular hindrance to integration efforts.
The present inventor proposed a method of forming non-reciprocal and reciprocal mode conversion regions in tandem in the waveguide using laser annealing to achieve the requisite localized control of the Ando, "Waveguide magneto-optical isolator fabricated by laser annealing", Appl. Phys. Lett. 53, 4-6, 1988). However, the precision required by the laser annealing makes it a costly fabrication process.
Another proposed method for forming the two regions in tandem involves providing the waveguide with a mirror, whereby non-reciprocal mode conversion is effected in a first region which extends as far as the mirror, utilizing a magneto-optic effect produced by a magnetization direction which is parallel to the direction of light propagation, and stress-induced optical anisotropy between the substrate and the waveguide is used to effect reciprocal mode conversion in a second region arranged at right angles to the propagation direction of light reflected by the mirror (H. Hemme et al.: "Optical isolator based on mode conversion in magnetic garnet films", Applied Optics 26(1987)3811). While this method facilitates magnetization parallel to the direction of light propagation in the first region, a special substrate is needed to achieve the required stress-induced anisotropy in the second region, and it is necessary to apply considerable stress to realize the desired degree of anisotropy. This makes it difficult to obtain good quality thin films for the waveguide and affects the longterm stability of the device.